The disclosure generally relates to an audio signal processing circuit and, more particularly, to the audio signal processing circuit for reducing the zero crossing distortion.
BD mode encoders are often utilized in the audio systems for performing audio signal processing operations so as to configure the load (e.g., a speaker) to generate the required sound. The BD mode encoder converts an audio signal into a corresponding inverted signal and a corresponding non-inverted signal to differentially represent the original audio signal. The inverted signal and the non-inverted signal are respectively modulated by the pulse width modulation (PWM) circuit to generate an inverted PWM signal and a non-inverted PWM signal. The inverted PWM signal and the non-inverted PWM signal are utilized to drive a full bridge power stage circuit so as to configure the speaker to generate the required sound. Some noise and even-order nonlinear signals may be eliminated by the BD mode encoder and not transmitted to the speaker. As a result, the audio system may generate good quality sounds.
The BD mode encoder, however, has the zero crossing distortion problem. Specifically, when the edge of the inverted PWM signal and the edge of the non-inverted PWM signal are too close, the circuits in the audio system are prone to generating noise and accordingly the speaker generates unpleasant sound. FIG. 1 shows an audio signal, the corresponding non-inverted PWM signal and the corresponding inverted PWM signal in the audio system. When the absolute value of the audio signal is larger, the difference between the pulse width of the inverted PWM signal and the pulse width of the non-inverted PWM signal is larger. Thus, the power stage circuit drives the speaker to generate a louder sound. When the absolute value of the audio signal is smaller, the difference between the pulse width of the inverted PWM signal and the pulse width of the non-inverted PWM signal is smaller. Thus, the power stage circuit drives the speaker to generate a smaller sound. For example, when the audio system is mute, the value of the audio signal is configured to be 0. The audio system generates an inverted PWM signal and a non-inverted PWM signal both of which have the same pulse width. In this situation, the edge of the pulse of the inverted PWM signal and the edge of the pulse of the non-inverted PWM signal are very close. As a result, the circuits in the audio system are prone to generating noise and accordingly the speaker generates unpleasant sound. Especially when the audio system is configured to generate a small sound or to be mute, the zero crossing distortion problem is more severe. Because the difference between the pulse width of the inverted PWM signal and the pulse of the non-inverted PWM signal is very small, the circuits in the audio system are prone to generating noise. Users are more likely to hear and be bothered by the unpleasant sound from the speaker when the audio system is configured to generate a small sounder or to be mute.
The approach disclosed in U.S. Pat. No. 6,373,336 utilizes a delay circuit in the audio system so as to delay one of the inverted PWM signal and the non-inverted PWM signal for a designated time interval. Accordingly, when the audio system is configured to generate a small sound or to be mute, the inverted PWM signal and the non-inverted PWM signal are separated by the designated time interval. Thus, the noise caused by the zero crossing distortion may be reduced.
In order to flexibly adjust the designated time interval in different audio systems or to avoid the inaccuracy of the designated time interval caused by the variation in manufacturing processes, complicated delay circuits and control circuits are necessary in the aforementioned audio systems. The complicated delay circuits, however, not only introduce additional noises and affect the performance of the audio system, but also increase the hardware design complexity and the production cost.